A) Field of the Invention
This invention relates to a solid state imaging device having a plurality of photoelectric conversion elements (photodiodes) in a light receiving region.
B) Description of the Related Art
As a solid state imaging device, a CCD solid state imaging device that transmits a signal charge using a charge coupled device (CCD), a MOS type solid state imaging device that outputs after amplifying an image signal from a photosensitive element with a MOS transistor and the like are well known. The photosensitive elements may be arranged in a tetragonal matrix wherein the elements are arranged in a fixed pitch in row and column directions or a honeycomb arrangement (Pixel Interleaved Arrangement) wherein the elements in the even number rows/columns and in the odd number rows/columns are shifted in a horizontal/vertical direction, for example, by about a half pitch of the elements.
In a case of a solid state imaging device equipped with an on-chip color filter, a color filter layer is formed on a semiconductor chip on which photosensitive elements and signal transmission parts are formed. In many cases, on-chip micro lenses are arranged over the color filter layer so that an incident light can be efficiently led into the photosensitive element.
A solid state imaging device equipped with more than two types of photodiodes receiving light of different sensitivity in one pixel is proposed (for example, refer to Japanese Patent Application 2002-016835 (Patent Document 1), Japanese Patent Application 2002-356813 (Patent Document 2), Japanese Patent Application Hei10-289052 (Patent Document 3)). High-sensitive photodiodes are formed in a relatively large region of the semiconductor chip, and low-sensitive photodiodes are formed in a relatively small region. When the high-sensitive photodiodes and the low-sensitive photodiodes which have different sensitivity are formed in the light receiving region of the solid state imaging device, an image having a wide dynamic range can be obtained.
In order to obtain an image having a wide dynamic range, output signals of the high-sensitive photodiode and the low-sensitive photodiode are synthesized. At that time, by adjusting spectral sensitivity of each type of the photodiode, it is preferable to avoid deterioration of a color S/N ratio after white balance correction.
In the patent document 1, a solid state imaging device having a structure wherein a fixed color filter is positioned over each photodiode is disclosed. Two types of photodiodes, each of which is formed with an n-type impurity layer in a p-type well, that is, the high-sensitive photodiode and the low-sensitive photodiode, have different impurity distributions, sizes and shapes, for example, an n-type impurity layer of the low-sensitive photodiode has a narrow doped region. Therefore, a narrow channel effect by a p-type isolation layer formed between a p-type well, the high-sensitive photodiode and the low-sensitive photodiode causes a shallow depletion layer of the low-sensitive photodiode, and sensitivity in a long wave side tends to decrease. Therefore, spectrum sensitivity between the high-sensitive photodiode and the low-sensitive photodiode tends to become imbalanced. In a solid state imaging device having a structure having a fixed colored color filter layer on an upper part of each photodiode, it is difficult that the low-sensitive photodiode and the high-sensitive photodiode have a same spectrum sensitivity. Although it is possible to adjust spectrum sensitivity by changing conditions such as an amount of dose and acceleration energy at a time of ion-implantation of impurities, other properties receive a bad influence.
In the solid-state imaging device, the incidence ray energy per unit area declines as it goes toward a peripheral area of a light receiving region. As a result, an amount of light will decrease at the peripheral area of the light receiving region, and that phenomenon is called illumination shading. Various methods for improving this illumination shading have been suggested. For example, in Japanese Laid-Open Patent Hei5-346556 (Patent Document 4), a method for controlling a decrease of light concentrating efficiency at a peripheral area of the solid-state imaging device by a so-called “micro-lens shifting” is disclosed. Also, in Japanese Laid-Open Patent 2002-198508 (Patent Document 5), a method for preventing shading of the incident light at a peripheral area of the light receiving region by shifting an opening position of a light shielding film toward the center of the solid-state imaging device is disclosed. Moreover, in Japanese Laid-Open Patent 2003-197897 (Patent Document 6), a method that the incident light certainly passes the opening by filling up a material with a high refractive index in the light shielding film opening is disclosed.
In recent years, by further increase of a number of pixels and miniaturization of a pixel size in a solid-state imaging device, in addition to the above-described illumination shading, color shading has become one of problems.